Low stress, water-clear zinc sulfide

ABSTRACT

The machinability of water-clear zinc sulfide articles produced by chemical vapor deposition and high temperature, high isostatic pressure (HIP) treatment is enhanced by extending the time over which the article is cooled following the HIP treatment. The resulting low stress, water-clear zinc sulfide articles can be more accurately finished/machined to precise shapes, such as are required in optical applications, than was previously possible.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Zinc sulfide is a durable material which is intrinsically transparent torelatively long electromagnetic wavelengths in the far-infrared region.These properties contribute to its use in applications which requireinfrared transmission capability such as in infrared detectors andmissile domes. Such zinc sulfide articles are typically produced bychemical vapor deposition (CVD) or hot pressing techniques. Thesetechniques result in forms which are generally opaque and notfunctionally transparent in the visible or near-infrared regions of theelectromagnetic spectrum. Hot-isostatic pressing (HIP) has been found tosufficiently improve the transparency of zinc sulfide articles in thevisible and near-ultraviolet regions that these forms can be used inapplications requiring multi-spectral capability, such as in tank andaircraft windows. However, difficulties in the final shaping/machiningof these windows have compromised the capability of providing them inthe precisely contoured shapes required for some applications, such asfor some optical components. These difficulties have been particularlypronounced with attempts to fabricate relatively large articles. Thisinvention relates to improvements in the CVD-Hipping process which havefacilitated the consistent fabrication of precisely shaped, low stress,water-clear zinc sulfide articles.

2. Description of Related Art

Chemical vapor deposition has been used to produce zinc sulfide in bulkform or in the form of a layered coating on a substrate. Typically suchproduction involves the reaction of H₂S with vaporized zinc in proximityto either a substrate or a mandrel box on which the zinc sulfidedeposits, such as is described in U.S. Pat. No. 5,686,195. The zincsulfide deposits produced by this CVD method generally exhibit manyinclusions and poor transmission in the visible and near-infraredregions of the electromagnetic spectrum. As described in U.S. Pat. No.4,944,900, the transmission properties in these regions can besubstantially improved by hot isostatic pressing (HIP) of the formproduced by CVD.

A moderately clear ZnS has been commercially produced by a two-stepprocess. First, elemental zinc vapors are reacted with hydrogen sulfideat a H₂S/Zn molar ratio of 1, a mandrel (substrate) temperature of 735°C. and an absolute pressure of 35 torr, in a CVD reactor. Zinc sulfideis deposited on the mandrel until a deposit of the desired thickness isproduced. The deposit is separated from the mandrel to provide a zincsulfide form which is then HIP treated for up to 100 hours at 900-1000°C. and pressures of 15,000-30,000 psi. A recent application, Ser. No.09/018,969 filed Feb. 5, 1998, describes an improvement of this processwhich reduced the number of visible inclusions present in the depositedform and provided a corresponding improved clarity of images viewedthrough articles prepared from these forms. The improved clarity of theproduct, referred to as water-clear zinc sulfide, has resulted in anexpanded range of applications including some which require physicalshaping/grinding to very precise final contours. Customers, however,have complained of difficulty accurately machining windows from 15-20inch forms of water-clear zinc sulfide to a specification which requireda figure of less than one tenth of a wave RMS (root mean square) at awavelength of 632.8 nm. Exceeding this specification resulted in productwindows which demonstrated unacceptable distortion of the transmittedimages.

It was suspected the machining difficulties were related to a high levelof stress birefringence, in the range of 80-240 nm/cm., which wasmeasured in large water-clear zinc sulfide forms. These high stressbirefringence values were generally measured near the edges of theforms, while relatively low values were measured near the forms'centers. Since considerable machining of the as deposited siliconcarbide form, to reduce both its thickness and its edges, is required toresult in the desired window shape, it was suspected such machining wasresponsible for the high stress birefringence values. Careful control ofthe machining and etching process provided a limited reduction of thetotal stress birefringence measured in forms; however, the stressbirefringence per unit of window thickness was not appreciably reduced.A further attempt to reduce the stress values involved repeating the HIPtreatment and controlling the cool down to less than 31° C. per hour.The second HIP treatment reduced the stress birefringence at the edgesfrom a value in excess of 100 nm/cm to about 70 nm/cm. While this secondHIP treatment demonstrated a reduction in the stress value, it was notsufficient to provide the required machinability. Providing a series ofHIP treatments to achieve a satisfactory level of stress birefringencewas not considered an economically viable approach, since each HIPtreatment requires heat soaking the form at high temperature and highpressure for up to 100 hours in a fairly large, high temperature rated,pressure vessel.

Accordingly, there is a need for forms of machinable, low stress,water-clear zinc sulfide, particularly relatively large forms, i.e.those having thicknesses greater than {fraction (1/2 )}inch and/or amaximum face dimension (length and/or width) greater than 10 inches.There is also a corresponding need for a process capable of consistentlyproducing such low stress, water-clear zinc sulfide forms.

SUMMARY OF THE INVENTION

The invention is an improvement of the process of producing water-clearzinc sulfide by chemical vapor deposition and hot isostatic pressingwhich permits the production of low stress, water-clear forms which arecapable of being formed/machined into precision windows and otheroptical articles. The invention also embraces the low stress,water-clear zinc forms/articles enabled by the improved process.

The improved process involves chemical vapor deposition of zinc sulfide,produced by the reaction of hydrogen sulfide with zinc vapor, on asubstrate and the subsequent removal of the zinc sulfide deposited formfrom the substrate. The form is then subjected to high temperature andhigh isostatic pressure (Hipping) for an extended period of time. At theconclusion of the high temperature treatment, the form is allowed tocool to room temperature under controlled conditions which maintain acooling rate of less than 50° C. per hour or, preferably, less than 31°C. Pressure is maintained on the cooling form at least until thetemperature is reduced to 500° C.

The improved low stress, water-clear zinc sulfide forms/articlesproduced by this process are characterized by stress birefringencevalues no greater than 40 nm/cm, and, preferably no greater than 20nm/cm.

BRIEF DESCRIPTION OF THE DRAWING

The drawing schematically illustrates a furnace in which the chemicalvapor deposition steps of the present inventive process can beconducted.

DETAILED DESCRIPTION OF THE INVENTION

Deposits of zinc sulfide are produced in a furnace like thatschematically illustrated in the FIGURE. The furnace 10 is enclosed in avertically oriented water cooled stainless steel vacuum chamber housing12. A graphite retort 14 containing molten zinc 15 and provided with afirst heating means, such as resistance and/or radiant heating elements,is provided near the bottom of the chamber 12. A rectangular graphitemandrel 16 is arranged above the zinc retort 14 with its interior inflow communication with the retort. Second heating means 18, such asresistance heaters, capable of heating the graphite mandrel are providedaround the mandrel's exterior. A gas injector 20 provides hydrogensulfide (H₂S) and an inert carrier gas to the lower portion of themandrel's interior. The gas exhaust 22 at the top of the housing 12 isoperatively connected to a filtration system (not shown) to removeparticulates, then to a vacuum source, such as a vacuum pump (not shown)and finally to a scrubber (not shown) to remove unreacted H₂S and anyother toxic products. The temperature of the mandrel is measured by athermocouple 24 touching the mandrel at its external surface. Thetemperature of the zinc in the retort is measured by averaging thetemperature measurements of two thermocouples, one 26 touching the upperportion of the retort's wall (above/near the level of molten zinc) andanother thermocouple 28 extending to the lower portion of the retort'swall (below the level of molten zinc).

In operation, elemental zinc is vaporized in the zinc retort 14 at atemperature greater than 575° C. The vaporized zinc is mixed withinjected H₂S and a carrier gas as they enter the mandrel 16 from theinjector 20. The mixed gases are caused to flow through the interior ofthe graphite mandrel wherein they contact the heated interior surface ofthe mandrel causing the zinc and H₂S to react forming ZnS on theinterior surfaces of the mandrel 16. The carrier gas and any gaseous orentrained reaction products are removed from the chamber through the gasexhaust 22 and are then processed through the filtration and scrubbingsystems. Once started, the process is continued until the desiredthickness of zinc sulfide is deposited on the graphite mandrel, whichtakes more than 15 hours and can take up to 1100 hours. Typically,deposition is continued for between 100 and 600 hours. When the desiredthickness is achieved, the gas flow through the gas injector 20 isdiscontinued, the first heating means is turned down, the second heatingmeans 18 is turned off, the chamber housing 12 is opened and thegraphite mandrel 16 is removed. The zinc sulfide sheets deposited on theinterior walls of the mandrel are then removed therefrom and cut intosheets of the desired size.

The sheets are machined to remove graphite contaminants on thesubstrate, or mandrel, side and are machined to smooth the depositionside. The sheets are then treated (Hipped) by a HIP process whichtypically subjects them to high temperature (greater than 700° C.,preferably 900° to 1000° C.) and isostatic high pressure (from 5,000 to30,000 psi, preferably from 15,000 to 30,000 psi) for an extended timeof up to 150 hours, typically 70 to 100 hours.

The HIP process involves wrapping the machined sheets in an inert foil,such as a platinum foil, and positioning the wrapped sheets in agraphite crucible in a conventional HIP furnace. The furnace is firstevacuated and then pressurized with an inert gas, such as argon. Heatingis begun and the temperature allowed to rise to its set point, where thetemperature and pressure stabilize and are maintained for the desiredextended treatment time. Upon completion of the desired treatment time,the heating is discontinued and the wrapped sheets allowed to cool. Thepressure is released after the temperature falls below 500° C.

Prior to this invention, the rate of cooling following HIP treatment wasgenerally set with concern for maximizing the throughput of the HIPfacility. Accordingly, the HIP treated forms were allowed to coolrelatively rapidly, i.e. at rates in excess of 100° C. per hour,particularly at the initiation of the cooling cycle. The presentinvention involves the discovery that by extending the cooling time bycontrolling the rate of cooling to less than 50° C. per hour, and,preferably, less than 31° C. per hour, forms having stress birefringencevalues no greater than 40 nm/cm, preferably, no greater than 20 nm/cm,and capable of final shaping/polishing as precisely shaped opticalcomponents, are produced.

While the improved process is advantageously used in the production ofany size form of low stress zinc sulfide, it is particularly beneficialwhen used to produce the relatively large forms which have previouslybeen the most difficult to finish in precisely contoured shapes, such asthe more precision shaped articles required in optical applications.Generally, these relatively large forms have a deposited thickness of ¼inch or greater, and/or they have a maximum face dimension (length orwidth) of 10 inches or greater. The improved process is particularlyimportant for the production of forms having a thickness of ¾ inch orgreater and/or a maximum face dimension of 20 inches or greater.

EXAMPLE NO. 1

Zinc sulfide forms were produced by chemical vapor deposition in afurnace similar to that illustrated in FIG. 1. Initially, the furnacewas flushed with an inert gas and pressure in the furnace was brought to35 torr. The mandrel 16 was brought to an initial temperature of 700° C.and zinc in the retort 14 was heated to a temperature in excess of 575°C. The flow of argon and hydrogen sulfide through injector 20 wasinitiated at flow rates of 113.1 slpm (standard liters per minute) Arand 9.3 slpm H₂S. The Zn vapor was gradually brought to its target flowrate of 12.43 slpm by increasing the zinc retort control temperature,measured near the top of the retort, from 640° C. to 660° C. over thefirst 38 hours of deposition. Over the same time period the temperaturemeasured near the bottom of the retort increased from 612° C. to 625°C., and the average of the two temperatures increased from 626° C.642.5° C. Thereafter, the retort control temperature increased, asrequired to maintain the Zn vapor flow rate of 12.43 slpm, until itreached 680° C. at hour 615 of the deposition. The mandrel temperaturewas ramped down from its initial 700° C. temperature to its final targetvalue of 670° C. over the first 20 hours of the deposition.

The deposition continued for 650 hours resulting in a 797.7 Kg ZnSdeposit on the mandrel. The deposit was formed into plates as it wasremoved from the mandrel. The plates were machined on the substrate sideto remove graphite contaminants and on the deposit side to smooth thesurface. They were then wrapped in platinum foil and vertically arrangedin a graphite crucible which was then loaded in a HIP chamber. Heatingof the crucible was initiated and an isostatic pressure of greater than14,000 psi was established in the HIP chamber by the time thetemperature reached 500° C. The temperature rose to 990° C. and thattemperature and a pressure of 15,000 psi was maintained for 90 hours.The crucible was then cooled to room temperature at a rate controlled tonot exceed 31° C. per hour. The pressure was reduced to atmosphericafter the temperature was reduced to less than 500° C.

After removal from the crucible, the plates were lapped and polished.Test samples (1 inch diameter by 10 mm thick), which were prepared fromthe same CVD deposit and which were included with the plates during theHIP treatment, were inspected for inclusions and determined to be ofexcellent quality with no inclusion greater than 0.1 mm in diameterdetected. Scatter values of the same samples were measured with ascatterometer at a half cone angle of 0.5 to 3 degrees from the beamdirection of a He-Ne laser (wavelength of 632.8 nm) source. The scattervalues varied in the range of 3.7 to 5.83 cm⁻¹. Visible transmissionvalues measured on two samples with a UV-vis spectrophotometer were inthe range of 56.3% to 59.1% for a 450 nm wavelength and in the range of62.4% to 64.8% for a 550nm wavelength.

Six plates produced during the run were characterized for stressbirefringence. Plates A, B, C and D were approximately 19 inches ×17inches ×0.85 inch and were intended for processing into 16 inch diameterwindows. Plates E and F were approximately 20 inches ×19 inches ×0.85inch and were intended for processing into 18 inch diameter windows.Measurements were made with a hand held Soleil-Babinet compensator. Themeasurements were taken at specified distances from the center alongseveral radii R extending from the center of the plate. The measurementswere averaged and reported in Table 1. The values reported under “Sides”are the average of six measurements, three along each of the shortersides. All of the stress birefringence values are ≦40 nm/cm. The dataestablishes that large, low stress, low scatter, high transmission,water-clear zinc sulfide plates were produced by the improved inventiveprocess.

TABLE 1 Stress Birefringence Measurements (nm/cm) Window ¼ R ½ R ¾ RFull R Sides Average A 3 4 6 11 9 6.6 B 2 2 2 7 13 5.2 C 2 7 8 16 15 9.6D 2 2 3 5 9 4.2 E 2 5 6 12 17 8.4 F 5 15 19 40 25 20.8

EXAMPLE NO. 2

A further deposit of zinc sulfide was prepared by chemical vapordeposition in a manner similar to the chemical vapor deposition ofExample No. 1. In this run the retort control temperature was ramped upfrom 645° to 665° C. and the temperature measured near the bottom of theretort was increased from 615° to 627° C. during the first 54 hours ofthe deposition. The mandrel temperature was ramped down from itsstarting temperature of 690° to 670° C. during the first 18 hours of thedeposition. The flow rate of zinc was established at 12.6 slpm and theH₂S/Zn molar ratio was controlled at 0.74. Otherwise, the conditionswere as in Example No. 1. The run continued for 650 hours.

One large rectangular form and several one and two inch diameter sampleswere prepared from the deposit and were HIP treated under the sameconditions as were used in Example No. 1, except the Hipping durationwas decreased to 70 hours and the treatment was conducted at a differentlocation.

The large form resulting from the HIP treatment was intended to providesufficient material to yield an 18 inch diameter window. It was polishedand characterized for stress birefringence by multiple measurements atdesignated locations spaced from the center of the form along severalradii R (in this case, R=9 inches) and at three points along its shortsides. The measurements were averaged and reported in Table 2. All ofthe averaged measurements are less than 20 nm/cm. The samplesdemonstrated forward scatter values and visible transmission valuesconsistent with the Example No. 1 values. Likewise, no inclusionsgreater than 0.1 mm were detected in the samples.

TABLE 2 Avg. Stress Number of Birefringence Measurement LocationMeasurements (nm/cm) Full R 8 13 ¾ R 8 5 ½ R 4 1 ¼ R 4 0 Short Sides 617 Average 6

A number of zinc sulfide forms produced prior to the present inventionby chemical vapor deposition and HIP treatment, with cool down from theHIP treatment at a rate initially in excess of 100° C./hour, werecharacterized for stress birefringence. Measurements were taken atpoints along two radii extending from the center of the formapproximately 90 degrees apart. The typical window thickness was 1.45cm. The results of the two data sets were averaged and reported in Table3. Each of the forms demonstrated stress birefringence values of 95nm/cm or greater.

TABLE 3 Stress Birefringence in 4 ZnS Forms Produced Prior To ThePresent Invention Measurement Location, Stress BirefringenceMeasurements (nm/cm) Radius Window Window Window Window (inch) No. 1 No.2 No. 3 No. 4 8.5 177 122 144 95 8.0 150 105 120 88 7.0 95 70 100 65 6.068 56 79 49 5.0 48 38 65 34 4.0 30 25 48 20 3.0 20 16 34 14 2.0 12 10 147 0.0 0 0 0 0 Average 60.4 44.8 61.5 38.4

Accordingly, it is believed that the inventive process has enabled, forthe first time, the production of low stress (i.e. less than 40 nm/cm),low scatter, high transmission, water-clear zinc sulfide.

The foregoing is provided to enable workers in the art to practice theinvention and to describe what is presently considered to be the bestmode of practicing the invention. The scope of the invention is definedby the following claims.

I claim:
 1. A method of preparing a low stress zinc sulfide formcomprising: forming a deposit of zinc sulfide on a substrate, separatingsaid deposit from said substrate as a zinc sulfide form, heating saidform to a temperature in excess of 700° C., providing an isostaticpressure of at least 5000 psi on said form, maintaining said temperatureand said pressure for at least 24 hours, and cooling said form from saidtemperature at a rate of less than 50° C. per hour.
 2. The method ofclaim 1, wherein said zinc sulfide deposit results from the reaction ofhydrogen sulfide with vaporized zinc.
 3. The method of claim 1, whereinsaid temperature is in the range of 900° to 1050° C.
 4. The method ofclaim 1, wherein said pressure is in the range of 15,000 to 30,000 psi.5. The method of claim 1, wherein said temperature and said pressure aremaintained for 50 to 150 hours.
 6. The method of claim 1, wherein saidzinc sulfide form is at least ¼ of an inch thick.
 7. The method of claim1, wherein said zinc sulfide form has at least one dimension in excessof 10 inches.
 8. The method of claim 7, wherein said zinc sulfide formhas at least one dimension in excess of 15 inches.
 9. The method ofclaim 1, wherein the stress birefringence values of said low stress zincsulfide form do not exceed 40 nm/cm.
 10. The method of claim 9, whereinsaid stress birefringence values do not exceed 20 nm/cm.
 11. The methodof claim 1, wherein said rate is less than 31° C. per hour.
 12. Themethod of claim 11, wherein said zinc sulfide form has at least onedimension in excess of 10 inches.
 13. The method of claim 11, whereinsaid zinc sulfide form is at least ¼ of an inch thick.
 14. A zincsulfide article produced by the method of claim 1, having at least onedimension in excess of ten inches and free of stress birefringencevalues exceeding 40 nm/cm.
 15. The zinc sulfide article of claim 14,further characterized by being free of inclusions greater than 0.1 mm indiameter.
 16. The zinc sulfide article of claim 14, furthercharacterized by 450 nm wavelength transmission values in the range of56 to 60%.
 17. The zinc sulfide article of claim 14, furthercharacterized by 550 nm wavelength transmission values in the range of62 to 65%.
 18. The zinc sulfide article of claim 14, furthercharacterized by having forward scatter values of 3.5 to 6.0 cm⁻¹ whenmeasured with a He-Ne laser.
 19. The zinc sulfide article of claim 14,having a thickness of at least ¼ inch.
 20. The zinc sulfide article ofclaim 14 having at least one dimension in excess of 15 inches.
 21. Azinc sulfide article according to claim 14 which is free of stressbirefringence values in excess of 20 nm/cm.